Marine iguanas are large lizard-like reptiles that are native to the Galapagos Islands, off the western coast of South America.

Charles Darwin observed these unusual creatures, the world's only lizards that depend on the ocean for their food.

Although they sometimes eat small crustaceans, their main meal is marine algae. Younger iguanas also eat their parents' faeces to gain useful gut bacteria that help them digest the algae.

Munching on algae

The researchers studied how the iguanas' feeding habits changed with temperature and the abundance of algae over the hot season from January to March, then the cooler season.

They looked at two sites close to the Charles Darwin Research Station on the island of Santa Cruz and another site about four kilometres away on the other side of the island.

They found that when the temperature warmed up the iguanas chewed faster. When it was cold, they munched their meals slower.

"They are little automata," said Shepherd. "The rate at which they bite is directly dependent on temperature."

Their feeding habits also depended on the tides and their sex. For example, the females fed in areas between high and low tide. But the males swam 50 to 100 metres from the shore and dived up to 15 metres to get a meal.

The iguanas preferred to feed at low tide, but wouldn't bother getting up if it was too early in the morning.

"They like to sleep in so they are an hour or two late, because they are waiting for the sun to warm them up," said Shepherd.

"They have to bake in the sun and get warm and energetic, then they rush in, feed, rush out and bake again."

If the low tide is unusually high, they will scorn their usual favourite red algae because it is submerged and difficult to reach, and feed instead on the green sea lettuce Ulva lobata.

"When the swell is up they have to adapt to that and rush out between swells, have a bite and rush back, or they hang on until the swell goes down," Shepherd said.

Watching how iguanas chew

Shepherd followed the iguanas' feeding habits for a year by sitting within one metre of them and counting the number of times they chewed their food over three minutes.

He then sampled the algae they were eating and measured the abundance of algal species to work out which food the iguanas preferred and which they avoided.

"If there are plenty of algae on the reef they'll graze once a day at low tide. If there is a shortage of food and the water is cold, they'll rush in and out and do this several times in one day."

He said that like humans, iguanas had an internal clock that tells them the time of day.

"If you blindfold them and lock them up in a cage, as some researchers have done, their internal clock still tells them when it is low tide."